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Abstract:

In a method for manufacturing chromium hydroxide according to the present
invention, chromium hydroxide having higher solubility in an acidic
aqueous solution than chromium hydroxide obtained by conventional
manufacturing methods can be obtained. The method is characterized by
simultaneously adding an aqueous solution of an inorganic alkali and an
aqueous solution containing trivalent chromium to an aqueous medium under
the condition of a reaction liquid temperature of 0° C. or more
and less than 50° C. to produce chromium hydroxide. The pH of the
reaction liquid while the aqueous solution containing trivalent chromium
and the aqueous solution of the inorganic alkali are added is preferably
maintained in the range of 7.0 to 12.

Claims:

1. A method for manufacturing chromium hydroxide, characterized by
simultaneously adding an aqueous solution of an inorganic alkali and an
aqueous solution comprising trivalent chromium to an aqueous medium under
the condition of a reaction liquid temperature of 0.degree. C. or more
and less than 50.degree. C. to produce chromium hydroxide.

2. The manufacturing method according to claim 1, wherein the addition of
the aqueous solution comprising trivalent chromium is performed so that
the amount of the trivalent chromium is not locally excessive with
respect to the amount of the alkali.

3. The manufacturing method according to claim 1, wherein the pH of a
reaction liquid while the aqueous solution comprising trivalent chromium
and the aqueous solution of the inorganic alkali are added is maintained
in the range of 7.0 to 12.

4. The manufacturing method of claim 1, wherein water, an aqueous
solution of a neutral salt, or ammonia water is used as the aqueous
medium.

5. The manufacturing method of claim 1, wherein after production of the
chromium hydroxide, filtration is performed, and the chromium hydroxide
is washed with water until the conductivity of a filtrate is 5 mS/cm or
less.

6. A method for manufacturing an aqueous solution of an inorganic acid
chromium (III) salt or an aqueous solution of an organic acid chromium
(III) salt, characterized by producing chromium hydroxide by a method
according to claim 1, and then dissolving the chromium hydroxide in an
aqueous solution of an inorganic acid or an aqueous solution of an
organic acid.

7. A method for manufacturing an aqueous solution comprising a source of
chromium (III), characterized by producing chromium hydroxide by a method
according to claim 1, and then dissolving the chromium hydroxide in an
aqueous solution of two or more acids.

8. An aqueous solution comprising a source of chromium (III) obtained by
a manufacturing method according to claim 6, used for metal surface
treatment or trivalent chromium chemical conversion treatment.

9. An aqueous solution comprising a source of chromium (III) obtained by
a manufacturing method according to claim 7, used for metal surface
treatment or trivalent chromium chemical conversion treatment.

10. The manufacturing method according to claim 2, wherein the pH of a
reaction liquid while the aqueous solution comprising trivalent chromium
and the aqueous solution of the inorganic alkali are added is maintained
in the range of 7.0 to 12.

11. The manufacturing method of claim 2, wherein water, an aqueous
solution of a neutral salt, or ammonia water is used as the aqueous
medium.

12. The manufacturing method of claim 3, wherein water, an aqueous
solution of a neutral salt, or ammonia water is used as the aqueous
medium.

13. The manufacturing method of claim 2, wherein after production of the
chromium hydroxide, filtration is performed, and the chromium hydroxide
is washed with water until the conductivity of a filtrate is 5 mS/cm or
less.

14. The manufacturing method of claim 3, wherein after production of the
chromium hydroxide, filtration is performed, and the chromium hydroxide
is washed with water until the conductivity of a filtrate is 5 mS/cm or
less.

15. The manufacturing method of claim 4, wherein after production of the
chromium hydroxide, filtration is performed, and the chromium hydroxide
is washed with water until the conductivity of a filtrate is 5 mS/cm or
less.

Description:

TECHNICAL FIELD

[0001] The present invention relates to a method for manufacturing
chromium hydroxide. The chromium hydroxide manufactured according to the
method of the present invention is useful for, for example, chromium
plating or metal surface treatment or trivalent chromium chemical
conversion treatment.

BACKGROUND ART

[0002] Chromium plating is used for decoration and for industrial use in
many industrial fields. Chromium plating is widely used as decoration
plating because it does not corrode in the air and does not lose luster.
Also, chromium plating is widely used for mechanical parts and the like
that require wear resistance because it has high hardness and a low
friction coefficient. A large amount of hexavalent chromium is used in a
plating liquid used for this plating. Since the effect of hexavalent
chromium on the human body is feared, hexavalent chromium must be reduced
to trivalent chromium under very strict conditions so that the hexavalent
chromium is not released in the environment in the treatment of the
plating waste liquid. Therefore, the development of a plating liquid
using trivalent chromium, which is chromium having low toxicity, instead
of hexavalent chromium, is desired.

[0003] For the plating liquid using trivalent chromium, for example,
Patent Document 1 describes a chromium plating liquid using a trivalent
chromium compound, such as chromium chloride, chromium sulfate, and
chromium sulfamate, as a plating liquid for decoration plating. However,
when an inorganic salt of trivalent chromium, such as chromium chloride
or chromium sulfate, is used as a source of chromium, chromium is
consumed by plating, while chloride ions or sulfate ions, which are the
counteranions of the chromium salt, remain in the plating liquid. From
the necessity to keep the liquid composition of the plating liquid
constant, the plating liquid is used by appropriately adding the source
of chromium in an amount corresponding to the consumed chromium, and
therefore, chloride ions or sulfate ions are accumulated in the plating
liquid. Therefore, finally, the liquid composition cannot be kept
constant, and thus, the total amount of the plating liquid is replaced by
a new plating liquid, and the spent plating liquid is treated as a waste
liquid.

[0004] As a method for solving this problem, Patent Document 2 proposes a
trivalent chromium plating method in which when trivalent chromium
plating is performed using a plating liquid containing chromium chloride
and ammonium chloride, part of the plating liquid is circulated in a
cooling apparatus, and part of the ammonium chloride is crystallized and
removed in this cooling apparatus to perform plating while controlling
the concentration of the ammonium chloride in the plating liquid.

[0005] Also, solving this problem by using, as a source of trivalent
chromium, chromium hydroxide, which is a compound without counteranion
accumulation, in the state of a water-containing gel is proposed (see
Patent Document 3). However, chromium hydroxide is generally insoluble in
water and has low solubility in an acidic aqueous solution used as an
ordinary plating liquid. Therefore, long-time stirring under heating is
required for the preparation of a plating liquid. Also, when the consumed
chromium is supplemented, a long time is required to dissolve supplied
chromium hydroxide. For these reasons, the plating work is interrupted
during that time, and problems occur in the preparation of the plating
liquid and the plating work.

[0006] The following methods (1) to (3) are known as conventional methods
for manufacturing chromium hydroxide (see Patent Documents 4 to 6).

Method (1):

[0007] A method in which a reducing agent is previously added to an
aqueous liquid containing chromate ions discharged in a chromium plating
step and the like to reduce the chromate ions in the liquid to trivalent
chromium ions, to which sodium hydroxide is added to obtain a precipitate
of chromium hydroxide (Patent Document 4). However, impurity ions, such
as sulfate ions, are often attached to chromium hydroxide obtained in
this manner, and the chromium hydroxide should be purified to be applied
to various uses. Patent Document 4 describes washing with water under the
condition of a pH of 9.5 or more.

Method (2):

[0008] Urea is added to an aqueous solution of sulfate containing
trivalent chromium. This solution is heated to a temperature in the range
of about 90° C. to the boiling point of the aqueous solution to
increase the pH of the liquid by the decomposition of the urea, and the
concentration of the sulfate ions in the aqueous solution is held at
about 1 mole/liter or less during this time to deposit and/or separate
basic chromium sulfate. Next, the produced basic chromium sulfate is
heated together with urea to the above temperature range or neutralized
with a hydroxide or carbonate of an alkali metal, an alkaline earth
metal, or ammonium to manufacture chromium hydroxide (Patent Document 5).
Patent Document 5 states that when chromium hydroxide is manufactured by
neutralization, a method starting from an aqueous solution of chromium
chloride is easy. But, there is no description of the solubility of the
obtained chromium hydroxide and addition order in neutralization.

Method (3):

[0009] An aqueous solution of a trivalent chromium salt is neutralized
with sodium hydroxide or ammonia water. A precipitate is separated from
the obtained slurry liquid of chromium hydroxide by filtration. The
separated precipitate is suspended in water to provide a slurry liquid.
This slurry liquid is passed through an ion exchange resin for separation
from the water-soluble impurities by adsorption (Patent Document 6).
Patent Document 6 describes a method in which an aqueous solution of
produced chromium sulfate or chromium chloride is neutralized with sodium
hydroxide or ammonia water, and a precipitate is separated from the
obtained slurry liquid of chromium hydroxide by filtration and then
washed to remove impurity ions. But, a method for adding sodium hydroxide
to an aqueous solution of chromium sulfate is used in the actual
manufacturing. Also, there is no description of the solubility of the
obtained chromium hydroxide.

[0018] It is an object of the present invention to provide a method for
manufacturing chromium hydroxide having high solubility.

Means for Solving the Problems

[0019] The present invention provides a method for manufacturing chromium
hydroxide characterized by simultaneously adding an aqueous solution of
an inorganic alkali and an aqueous solution containing trivalent chromium
to an aqueous medium under the condition of a reaction liquid temperature
of 0° C. or more and less than 50° C. to produce chromium
hydroxide.

[0020] Also, the present invention provides a method for manufacturing an
aqueous solution of an inorganic acid chromium (III) salt or an aqueous
solution of an organic acid chromium (III) salt characterized by
producing chromium hydroxide by the method mentioned earlier, and then
dissolving the chromium hydroxide in an aqueous solution of an inorganic
acid or an aqueous solution of an organic acid.

Advantages of the Invention

[0021] According to the present invention, chromium hydroxide having
higher solubility in an acidic aqueous solution than chromium hydroxide
obtained by conventional manufacturing methods can be obtained. By using
the chromium hydroxide manufactured by the method of the present
invention as a source of trivalent chromium, the time of the preparation
of a trivalent chromium plating liquid can be reduced, and adverse effect
on a plating coating due to undissolved chromium hydroxide can be
prevented. Also, when a liquid containing trivalent chromium, using the
chromium hydroxide manufactured by the method of the present invention,
is used for chromium plating or metal surface treatment or trivalent
chromium chemical conversion treatment, the counteranions of the source
of trivalent chromium are not accumulated in the plating liquid and the
like, and therefore, it is easy to keep the composition of the plating
liquid and the like constant. Also, the time of the preparation of the
plating liquid and the like is significantly reduced, and therefore, the
effect on related industries is large.

BEST MODE FOR CARRYING OUT THE INVENTION

[0022] One characteristic of the manufacturing method of the present
invention is simultaneous addition of an aqueous solution of an inorganic
alkali and an aqueous solution containing trivalent chromium. The present
inventors have known that by simultaneously adding these aqueous
solutions to an aqueous medium, unexpectedly, chromium hydroxide having
high solubility in an acidic aqueous solution can be obtained. On the
other hand, in conventional methods for manufacturing chromium hydroxide,
for example, the manufacturing methods described in Patent Documents 4
and 6, simultaneous addition is not used, and instead, an alkali, such as
sodium hydroxide, is added to the aqueous solution containing trivalent
chromium to produce chromium hydroxide. Chromium hydroxide obtained by
this method has poor solubility in an acidic aqueous solution. In the
following description, chromium means trivalent chromium unless otherwise
specified.

[0023] The aqueous solution of the inorganic alkali and the aqueous
solution containing trivalent chromium are substantially continuously
added to the aqueous medium. Substantially continuously is intended to
allow a case where the addition is unavoidably temporarily discontinuous
due to a fluctuation in manufacturing conditions, and the like.

[0024] In the simultaneous addition of the aqueous solution of the
inorganic alkali and the aqueous solution containing trivalent chromium,
both aqueous solutions are substantially simultaneously added at the
start of operation, though the addition of the aqueous solution of the
inorganic alkali may precede the addition of the aqueous solution
containing trivalent chromium, or on the contrary, the addition of the
aqueous solution containing trivalent chromium may precede the addition
of the aqueous solution of the inorganic alkali, to the extent of not
impairing the effect of the present invention. At the completion of
operation, similarly, the addition of both aqueous solutions are
substantially simultaneously completed, but the completion of the
addition of the aqueous solution of the inorganic alkali may precede the
completion of the addition of the aqueous solution containing trivalent
chromium, or on the contrary, the completion of the addition of the
aqueous solution containing trivalent chromium may precede the completion
of the addition of the aqueous solution of the inorganic alkali, to the
extent of not impairing the effect of the present invention.

[0025] The aqueous solution of the inorganic alkali and the aqueous
solution containing trivalent chromium are simultaneously added to the
aqueous medium. The aqueous medium used in the present invention
preferably has a pH in the neutral range to the alkaline range. It is
also possible to use an aqueous medium having a pH in the acidic range,
but considering that the produced chromium hydroxide has good solubility,
it is advantageous to use an aqueous medium having a pH in the neutral
range to the alkaline range.

[0026] For example, water (pure water) and an aqueous solution of a
neutral salt can be used as the aqueous medium having a pH in the neutral
range. For example, sodium chloride and the like can be used as the
neutral salt. Generally, the concentration of the neutral salt is
preferably up to about 1 mol/l. For example, ammonia water can be used as
the aqueous medium having a pH in the alkaline range. Generally, the
concentration of the ammonia water is preferably up to about 0.01 mol/l.
Also, the aqueous medium can contain a water-soluble organic solvent,
such as lower alcohol, as required, whether its pH is in the neutral
range or the alkaline range. Among these aqueous media, water (pure
water) is preferably used from the viewpoint that the mixing of
unnecessary chemical species can be prevented in the preparation of a
chromium plating liquid and the like.

[0027] The solubility of the produced chromium hydroxide is also affected
by the temperature of the reaction liquid, in addition to simultaneously
adding the aqueous solution of the inorganic alkali and the aqueous
solution containing trivalent chromium. The reaction liquid herein is a
liquid obtained by adding the aqueous solution of the inorganic alkali
and the aqueous solution containing trivalent chromium to the aqueous
medium. The temperature of the reaction liquid should be 0° C. or
more and less than 50° C. If the temperature of the reaction
liquid is 50° C. or more, the produced chromium hydroxide becomes
an aggregate or massive easily, and therefore, chromium hydroxide having
high solubility is not obtained. If the temperature of the reaction
liquid is less than 0° C., the trivalent chromium salt and/or the
inorganic alkali may be deposited. When the temperature of the reaction
liquid is 10° C. or more and less than 50° C., particularly
10 to 40° C., chromium hydroxide having high solubility is more
easily obtained, and therefore, such temperature is preferred.

[0028] The reaction of the aqueous solution of the inorganic alkali and
the aqueous solution containing trivalent chromium is neutralization
reaction, and therefore, chromium hydroxide having the desired properties
is obtained by mixing both aqueous solutions in the aqueous medium.
During the reaction with the simultaneous addition, preferably, the
reaction liquid is stirred to allow reaction to occur uniformly and
promote reaction. If the stirring is insufficient, the amount of the
trivalent chromium may be locally excessive with respect to the amount of
the alkali in the reaction liquid. Chromium hydroxide produced in such a
state has poor solubility in an acidic aqueous solution. Therefore, it is
important to perform the addition of the aqueous solution containing
trivalent chromium so that the amount of the trivalent chromium is not
locally excessive with respect to the amount of the alkali. From this
viewpoint, it is preferred to adjust the stirring conditions so that the
local occurrence of stagnant portions is avoided and uniform mixing can
be performed. The state in which the amount of the trivalent chromium is
locally excessive with respect to the amount of the alkali refers to, for
example, a state in which the aqueous solution of the inorganic alkali is
added to the aqueous solution containing trivalent chromium, as described
in Patent Documents 4 and 6.

[0029] The concentration, addition speed, addition ratio, and the like of
the aqueous solution of the inorganic alkali and the aqueous solution
containing trivalent chromium are not particularly limited, but in terms
of obtaining chromium hydroxide having high solubility, it is preferred
to adjust these according to the ability of the stirrer and the
manufacturing scale so that nonuniform mixing does not occur during the
reaction. For preferred concentration, the concentration of hydroxide
ions in the aqueous solution of the inorganic alkali is 1 to 50% by
weight, particularly 5 to 30% by weight, and the concentration of
trivalent chromium in the aqueous solution containing trivalent chromium
is 1 to 40% by weight, particularly 3 to 20% by weight. When 1 liter, for
example, of the aqueous medium is used, preferred addition speed is 2 to
150 ml/min, particularly 10 to 100 ml/min, for the aqueous solution of
the inorganic alkali, and 5 to 300 ml/min, particularly 10 to 200 ml/min
for the aqueous solution containing trivalent chromium, on the condition
that the concentration is in the above-described range. For the addition
ratio, that is, the ratio of addition speed, the addition speed of the
aqueous solution containing trivalent chromium is 0.1 to 20 times,
particularly 0.5 to 10 times, the addition speed of the aqueous solution
of the inorganic alkali, on the condition that the concentration and the
addition speed are in the above-described range.

[0030] It is preferred to adjust the concentration, addition ratio, and
addition speed of the aqueous solution of the inorganic alkali and the
aqueous solution containing trivalent chromium so that the pH of the
reaction liquid while these aqueous solutions are added is maintained at
7.0 to 12, particularly 7.5 to 10. By maintaining the pH during the
reaction in this range, chromium hydroxide having the target solubility
can be successfully manufactured.

[0031] A water-soluble trivalent chromium salt can be used, without
particular limitation, as the source of chromium in the aqueous solution
containing trivalent chromium. Such a salt includes, for example,
chromium chloride, chromium sulfate, chromium ammonium sulfate, chromium
potassium sulfate, chromium formate, chromium fluoride, chromium
perchlorate, chromium sulfamate, chromium nitrate, and chromium acetate.
One of these salts can be used, or two or more of these salts can be used
in combination. These salts may be used in the state of an aqueous
solution or in the state of a powder. For example, "35% Liquid Chromium
Chloride" and "40% Liquid Chromium Sulfate" (product names) manufactured
by Nippon Chemical Industrial Co., Ltd., and commercially available
chromium chloride (crystalline product) can be used. Among these salts,
chromium chloride and chromium sulfate are preferably used in terms of no
residual organic substance, easy drainage treatment, and economy.

[0032] As the aqueous solution containing trivalent chromium, one obtained
by reducing hexavalent chromium in an aqueous solution containing
hexavalent chromium to trivalent chromium can also be used. For example,
an aqueous solution obtained by passing a sulfurous acid gas through an
aqueous solution of dichromate to reduce hexavalent chromium to trivalent
chromium can be used. Alternatively, an aqueous solution obtained by
adding sulfuric acid to an aqueous solution of dichromic acid to reduce
hexavalent chromium to trivalent chromium with an organic substance can
also be used.

[0033] Alkali metal hydroxide, such as sodium hydroxide and potassium
hydroxide, ammonia, and the like can be used as the inorganic alkali used
in the aqueous solution of the inorganic alkali simultaneously added with
the aqueous solution containing trivalent chromium. If an organic alkali
is used instead of the inorganic alkali, the chromium may remain in the
filtration waste liquid after the production of chromium hydroxide
because the organic alkali forms a water-soluble complex salt with the
chromium. Therefore, the use of the organic alkali should be avoided.
When particularly alkali metal hydroxide is used among the
above-described inorganic alkalis, the good solubility of chromium
hydroxide in an acidic aqueous solution is maintained for a long period,
and therefore, the use of alkali metal hydroxide is preferred.

[0034] When chromium hydroxide is produced by simultaneously adding the
aqueous solution containing trivalent chromium and the aqueous solution
of the inorganic alkali, the slurry is filtered to separate the chromium
hydroxide as a solid, and the chromium hydroxide is washed. A usual
method can be used for the filtration. For example, suction filtration
using a Buchner funnel can be performed. The washing after the filtration
is performed using water. For example, the washing can be performed by
adding water to the cake on the Buchner funnel for repulping, and further
performing suction filtration, and the like. The washing is preferably
performed until the conductivity of the filtrate is, for example, 5 mS/cm
or less. A high conductivity of the filtrate means that a large amount of
by-product salts derived from the raw materials remain in the chromium
hydroxide after the washing. Such by-product salts should be removed as
much as possible because when the chromium hydroxide is used as a source
of chromium in a trivalent chromium plating liquid, the by-product salts
are accumulated in the plating liquid. Therefore, the washing is
preferably performed until the conductivity of the filtrate is the above
value or less. Also, the filtration and the washing are preferably
performed at a low temperature of preferably 0 to 50° C., more
preferably 20 to 40° C., because the olation and oxo synthesis of
chromium, and the resulting production of a poorly soluble substance can
be prevented.

[0035] After the washing, the chromium hydroxide is dried to be in a
powder state, or water is added to the chromium hydroxide to provide a
slurry having a predetermined concentration.

[0036] It is preferred to add a reducing agent during the above reaction,
or after the completion of the reaction. Thus, even if the reaction
liquid or the chromium hydroxide is placed in an oxidation atmosphere
during the reaction or during storage (during storage in a slurry state),
reoxidation can be prevented, and therefore, the production of hexavalent
chromium can be prevented. Particularly, it is preferred to add a
reducing agent after the completion of the reaction, from the viewpoint
that reoxidation can be surely prevented. Organic or inorganic reducing
agents conventionally used in the art can be used, without particular
limitation, as the reducing agent. For example, monohydric alcohol, such
as methyl alcohol and propyl alcohol, and dihydric alcohol, such as
ethylene glycol and propylene glycol, are suitably used as the organic
reducing agent. Other organic reducing agents include monosaccharides,
such as glucose, disaccharides, such as maltose, polysaccharides, such as
starch, and the like. The inorganic reducing agents include, for example,
hydrazine, hydrogen peroxide, and the like.

[0037] The chromium hydroxide manufactured according to the method of the
present invention (hereinafter also simply referred to as "the chromium
hydroxide of the present invention") is characterized by being insoluble
or poorly soluble in pure water, but having high solubility in an acidic
aqueous solution (for example, an acidic aqueous solution having a pH of
3 or less). Chromium hydroxide having such characteristics is specified
by its degree of aggregation and particle diameter. Specifically, the
chromium hydroxide of the present invention is in the form of fine
particles and has a low degree of aggregation. In the present invention,
the degree of aggregation is defined by MV/D. MV represents a volume
average particle diameter measured by a particle size distribution
measurement apparatus, and D represents an average particle diameter
measured from a scanning electron microscope (SEM) image. According to
this definition, a larger value of the degree of aggregation means higher
aggregation properties (that is, the particles aggregate).

[0038] A specific method for measuring the degree of aggregation is as
follows. Produced chromium hydroxide is sufficiently dispersed in water
by a household mixer or the like, and then, the volume average particle
diameter (MV) is measured by a laser diffraction scattering type particle
size distribution measurement apparatus. Also, the particle diameter of
200 primary particles of the chromium hydroxide is measured with an SEM
image, and the average value is the average particle diameter D with the
SEM image. When the primary particle is not spherical, the maximum length
across the particle is the particle diameter. A value obtained by
dividing MV measured in this manner by D is the degree of aggregation.

[0039] In the chromium hydroxide of the present invention, the degree of
aggregation calculated based on the above definition is preferably 10 or
more and less than 70. If the degree of aggregation of the chromium
hydroxide is more than 70, the solubility in an acidic aqueous solution
tends to decrease. In the chromium hydroxide manufactured according to
the method of the present invention, as the degree of aggregation becomes
lower, the solubility in an acidic aqueous solution becomes higher. From
this viewpoint, a smaller value of the degree of aggregation of the
chromium hydroxide is preferred. Since there is a limit to the lower
limit value of the degree of aggregation attainable by the manufacturing
method of the present invention, the lower limit value of the degree of
aggregation of the chromium hydroxide is defined as 10 in the present
invention. Particularly, when the degree of aggregation is 10 to 60,
change over time is reduced, and better solubility can be kept, and
therefore, such degree of aggregation is more preferred.

[0040] The chromium hydroxide of the present invention is in the form of
fine particles such that the average particle diameter D of the primary
particles is preferably 40 to 200 nm, and more preferably 50 to 100 nm.
Chromium hydroxide having such a particle diameter satisfies the
above-described degree of aggregation, and therefore, the solubility in
an acidic aqueous solution is improved. If the average particle diameter
D of the primary particles is less than 40 nm, the electrostatic
attraction is strong, aggregation occurs easily, and the solubility
decreases. If the average particle diameter D of the primary particles is
more than 200 nm, the specific surface area decreases, and portions
reacted with the acid decreases, and therefore, the solubility decreases.

[0041] The particle shape of the chromium hydroxide of the present
invention is not particularly limited and can be a shape, for example, a
spherical shape and a massive shape.

[0042] The chromium hydroxide of the present invention is generally in the
state of a dry powder or in the state of a slurry of the chromium
hydroxide suspended in water. In terms of increasing the solubility in an
acidic aqueous solution, the chromium hydroxide is preferably
continuously in the state of a slurry from immediately after it is
manufactured according to the method of the present invention. Components
other than the chromium hydroxide may be contained or may not be
contained in the slurry. When components other than the chromium
hydroxide are contained in the slurry, the components include Na, K, Cl,
SO4, NH4, and the like. When the slurry is used as a
supplementary liquid for a plating liquid and the like used for chromium
plating or metal surface treatment or trivalent chromium chemical
conversion treatment, the slurry preferably contains substantially no
impurity ions to prevent the accumulation of unnecessary ions due to
supplement. The "impurity ions" in this specification mean ions other
than H.sup.+ and OH.sup.- ions. "Contains substantially no" means that
impurity ions are not intentionally added during the preparation of the
chromium hydroxide and the preparation of the slurry using the chromium
hydroxide, and is intended to allow a slight amount of impurity ions
unavoidably mixed. Therefore, in addition to pure water and ion exchange
water, tap water, industrial water, and the like containing substantially
no impurity ions may be used as water used for the preparation of the
chromium hydroxide and the preparation of the slurry using the chromium
hydroxide.

[0043] As already described, the chromium hydroxide of the present
invention has high solubility in an acidic aqueous solution (for example,
an acidic aqueous solution having a pH of 3 or less). Moreover, its
solubility is maintained even after long-term storage. On the other hand,
conventionally obtained chromium hydroxide changes over time during
long-term storage, and easily transitions to hydroxide poorly soluble in
an aqueous solution of an acid or an alkali. The cause is not exactly
clear, but is considered to be that the chromium hydroxide transitions to
a poorly soluble form due to the olation and oxo synthesis of chromium.
Therefore, when a chromium plating liquid is prepared, stirring must be
performed for a long time until chromium hydroxide is completely
dissolved.

[0044] High solubility in this specification means that when chromium
hydroxide corresponding to a Cr content of 1 g is added to 1 liter of an
aqueous solution of hydrochloric acid having a pH of 0.2 at a temperature
of 25° C., the chromium hydroxide is completely dissolved within
30 minutes. The dissolution of the chromium hydroxide is visually
determined. The time of the dissolution of the chromium hydroxide is time
until the liquid becomes transparent.

[0046] The chromium hydroxide manufactured according to the method of the
present invention can be added to and dissolved in an aqueous solution of
an inorganic acid or an aqueous solution of an organic acid, in a powder
state, or by adding water to the chromium hydroxide to be in a slurry
state, thereby, an aqueous solution of an inorganic acid chromium (III)
salt or an organic acid chromium (III) salt is obtained. The
concentration and used amount of the chromium hydroxide and the aqueous
solution of the inorganic acid or the aqueous solution of the organic
acid can be appropriately determined according to the type (composition
formula) of the target inorganic acid chromium salt or organic acid
chromium salt, and the target concentration of the target inorganic acid
chromium salt or organic acid chromium salt in the final aqueous
solution.

[0047] In terms of dissolving the chromium hydroxide easily and surely,
the aqueous solution of the inorganic acid or the aqueous solution of the
organic acid preferably has low pH, specifically, preferably a pH of 2 or
less, and more preferably a pH of 1.5 or less. The concentration of the
inorganic acid or the organic acid in the aqueous solution of the
inorganic acid or the aqueous solution of the organic acid is preferably
in the range of 1 to 50% by weight, particularly 5 to 50% by weight.
Also, in terms of dissolving the chromium hydroxide easily and surely, it
is preferred to use the chromium hydroxide corresponding to 1 g or less
of Cr, with respect to 1 liter of the aqueous solution of the inorganic
acid or the aqueous solution of the organic acid.

[0048] The dissolution of the chromium hydroxide in the aqueous solution
of the inorganic acid or the aqueous solution of the organic acid is
preferably performed at 25 to 90° C.

[0049] The inorganic acid chromium salt obtained in this manner includes
chromium hydrochloride, chromium nitrate, chromium phosphate, chromium
sulfate, chromium fluoride, and the like. These inorganic acid chromium
salts may be basic salts. For example, chromium nitrate is a compound
represented by the composition formula Cr(OH)x(NO3)y, wherein
0≦x≦2, 1≦y≦3, and x+y=3, and the compound
also includes basic chromium nitrate, such as
Cr(OH)0.5(NO3)2.5, Cr(OH)(NO3)2, and
Cr(OH)2(NO3), in addition to chromium nitrate, which is a
normal salt represented by Cr(NO3)3.

[0050] The organic acid chromium salt is a compound represented by the
general formula Crm(Ax)n. In the above general formula, A
represents a residue obtained by removing a proton from an organic acid.
A has a negative charge. x represents the charge of A (negative charge).
m and n each represents an integer satisfying 3m+xn=0.

[0051] The organic acid in the organic acid chromium salt is represented
by R(COOH)y, wherein R represents an organic group, a hydrogen atom,
or a single bond or a double bond; and y represents the number of
carboxyl groups in the organic acid and is an integer of 1 or more,
preferably 1 to 3. A in the above general formula is represented by
R(COO.sup.-)y. When R is an organic group, the organic group is
preferably an aliphatic group having 1 to 10 carbon atoms, particularly 1
to 5 carbon atoms. This aliphatic group may be substituted with other
functional groups, for example, a hydroxyl group. Both of a saturated
aliphatic group and an unsaturated aliphatic group can be used as the
aliphatic group.

[0052] Also, the chromium hydroxide manufactured according to the method
of the present invention can be added to and dissolved in an aqueous
solution of two or more acids, in a powder state, or by adding water to
the chromium hydroxide to be in a slurry state, to provide an aqueous
solution containing a source of chromium (III). The concentration and
used amount of the chromium hydroxide and the aqueous solution of the
acids, the combination of the acids used, and the compounding ratio of
the acids can be appropriately determined according to the type of the
target source of chromium (III), and the target concentration of the
target source of chromium (III) in the final aqueous solution.

[0053] The type of the aqueous solution of the acids in which the chromium
hydroxide is dissolved includes a combination of organic acids, a
combination of inorganic acids, or a combination of an organic acid and
an inorganic acid. The organic acid and the inorganic acid that can be
used include those similar to those described previously.

[0054] The method for manufacturing an aqueous solution of two or more
acids containing a source of chromium (III) according to the present
invention should follow the method for manufacturing an aqueous solution
of an inorganic acid chromium salt or an organic acid chromium salt
described above, and therefore, detailed description of the manufacturing
method is omitted here. The outline is described. For example, the
following methods 1) to 3) can be used for the dissolution of chromium
hydroxide in an aqueous solution of acids, but the dissolution is not
limited to these methods.

[0055] 1) A method in which an aqueous solution of acids in which the
desired two or more acids are previously dissolved is prepared, and
chromium hydroxide is added to the aqueous solution to dissolve the
chromium hydroxide in the acid solution;

[0056] 2) A method in which an acid of one component among the desired
acids is previously appropriately selected, next, the selected acid is
dissolved in water to prepare an aqueous solution of the acid, next,
chromium hydroxide is added to the obtained aqueous solution of the acid
to perform primary dissolution treatment, and an acid (acids) of the
remaining component(s) is added to the obtained aqueous solution to
perform secondary dissolution treatment; or

[0057] 3) A method in which an aqueous solution of acids in which part of
the required amount of the desired two or more acids is previously
dissolved in water is prepared, next, chromium hydroxide is added to the
obtained aqueous solution of the acids to perform primary dissolution
treatment, and the remaining amount of the acids are added to the
obtained aqueous solution to perform secondary dissolution treatment to
dissolve the chromium hydroxide.

[0058] The source of chromium (III) of the present invention obtained in
this manner is a complex chromium (III) salt having two or more acid
radicals bonded to chromium, represented by the following formula. The
type of acids bonded to chromium may be selected from a combination of
organic acids, a combination of inorganic acids, or both of an organic
acid and an inorganic acid.

Crk(OH)l(A1x1)m1(A2x2)-
m2 . . . (Anxn)mR [Formula 1]

wherein A1, A2, . . . , An represent two or more acid
residues obtained by removing a proton from an acid selected from the
group consisting of an inorganic acid and an organic acid. When
phosphoric acid is used as the acid, the above acid residue is
represented by H2PO4.sup.- and/or HPO42-. x1,
x2, . . . , xn represent a charge. However, A1, A2, .
. . , An are not the same acid radical. k, l, m1, m2, . .
. , mn represent a real number satisfying
3k-l+x1m1+x2m2+ . . . +xnmn=0.)

[0059] In the above formula, the ratio of the above H2PO4and HPO42- when phosphoric acid is used as the acid is
arbitrarily changed by the reaction conditions, the raw material system,
and the like.

[0060] The chromium hydroxide manufactured according to the method of the
present invention has high solubility in an acidic aqueous solution as
described above and is therefore useful as, for example, a source of
trivalent chromium in chromium plating or a metal surface treatment
liquid or a trivalent chromium chemical conversion treatment liquid using
trivalent chromium, as described below. By using the chromium hydroxide
of the present invention as a source of trivalent chromium, the time of
the preparation of the plating liquid and the treatment liquid can be
reduced. Also, undissolved chromium hydroxide is not present in the
plating liquid and the treatment liquid, and therefore, a good-quality
plating coating and trivalent chromium chemical conversion coating can be
formed. The "trivalent chromium chemical conversion treatment" in this
specification refers to treatment in which a material to be treated is
brought into contact with an aqueous solution containing a trivalent
chromium salt as the main component to chemically produce a coating
containing trivalent chromium on the material to be treated.

[0061] The present invention also provides a liquid containing trivalent
chromium, using the above-described chromium hydroxide having high
solubility as a source of chromium. The liquid containing trivalent
chromium according to the present invention is used for final finish for
decoration and for industrial trivalent chromium plating. Also, the
liquid containing trivalent chromium is used for the surface treatment of
various metals, such as plating provided as a layer on nickel plating.
Further, the liquid containing trivalent chromium is used for the
trivalent chromium chemical conversion treatment of zinc plating, tin
plating, and the like. In other words, the liquid containing trivalent
chromium according to the present invention can be a trivalent chromium
plating liquid and a trivalent chromium chemical conversion treatment
liquid. In the following description, these liquids are collectively
called "a plating liquid and the like" unless otherwise specified.

[0062] When the liquid containing trivalent chromium according to the
present invention is used as a trivalent chromium plating liquid, the
trivalent chromium plating liquid contains trivalent chromium derived
from the above-described chromium hydroxide, and other components,
including an organic acid and the like. When the liquid containing
trivalent chromium according to the present invention is used as a
treatment liquid for trivalent chromium chemical conversion treatment,
the treatment liquid uses the above-described chromium hydroxide as a
source of chromium and can further contain a cobalt compound, a silicon
compound, a zinc compound, various organic acids, and the like.

[0063] The cobalt compound used for the above trivalent chromium chemical
conversion treatment liquid includes cobalt chloride, cobalt nitrate,
cobalt sulfate, cobalt phosphate, cobalt acetate, and the like. One of
these can be used, or two or more of these can also be mixed and used.
The silicon compound includes colloidal silica, sodium silicate,
potassium silicate, and lithium silicate. One of these silicon compounds
can be used, or two or more of these silicon compounds can also be mixed
and used. The zinc compound includes zinc chloride, zinc sulfate, zinc
nitrate, zinc oxide, zinc carbonate, zinc phosphate, zinc acetate, and
the like. One of these zinc compounds can be used, or two or more of
these zinc compounds can also be mixed and used. The organic acid
includes oxalic acid, malonic acid, succinic acid, citric acid, adipic
acid, tartaric acid, malic acid, glycine, and the like. It is considered
that since these exhibit chelating action, the trivalent chromium can be
held in the plating liquid in a stable form.

[0064] The above trivalent chromium chemical conversion treatment liquid
preferably contains 0.005 to 1.0 mole/liter, for example, of chromium.
The molar ratio of the organic acid to the chromium is preferably 1 to 5
moles with respect to 1 mole of the chromium.

[0065] The present invention also provides, in addition to the
above-described plating liquid and the like, a supplementary liquid for a
plating liquid and the like used for chromium plating or metal surface
treatment or trivalent chromium chemical conversion treatment. This
supplementary liquid comprises a slurry containing the above-described
chromium hydroxide. This slurry preferably contains no impurity ions as
described above. In metal surface treatment, trivalent chromium chemical
conversion treatment, and the like, inorganic anions, for example,
sulfate ions, nitrate ions, and chloride ions, are not taken in the
coating and remain in the liquid. Therefore, when the source of chromium
is additionally poured into the plating liquid and the like, inorganic
anions that are the counteranions of the source of chromium are gradually
accumulated in the plating liquid and the like, and the composition of
the plating liquid and the like is changed. On the other hand, the
supplementary liquid comprising the slurry containing the above-described
chromium hydroxide does not contain these anions, and therefore, even if
the supplementary liquid is additionally poured into the plating liquid
and the like as a source of chromium, the change in the composition of
the plating liquid and the like is small. As a result, the plating liquid
and the like can be used for a long period without frequently renewing
the plating liquid and the like.

[0066] The type of the plating liquid and the like to which the source of
chromium is supplied by the above supplementary liquid is not
particularly limited, and a plating liquid and the like containing
trivalent chromium conventionally used can be used.

[0067] A suitable amount of the supplementary liquid of the present
invention is added to the plating liquid and the like according to the
extent of the consumption of chromium ions in the plating liquid and the
like, while plating and trivalent chromium chemical conversion treatment
are performed. The addition may be continuous or intermittent.

[0068] The present invention has been described based on the preferred
embodiment of the present invention. However, the present invention is
not limited to the above embodiment, and various modifications can be
made within the common sense of those skilled in the art. Such
modifications fall within the scope of the present invention.

EXAMPLES

[0069] The present invention will be specifically described below by way
of Examples. "%" means "% by weight" unless otherwise specified.

Example 1

[0070] 140 g of a 10% aqueous solution of sodium hydroxide, and a 7%
aqueous solution of chromium chloride obtained by adding 220 g of water
to 55 g of a 35% aqueous solution of chromium chloride (manufactured by
Nippon Chemical Industrial Co., Ltd.) for dilution were each placed in a
container and prepared. Next, the aqueous solution of sodium hydroxide
was adjusted to 20° C., and the aqueous solution of chromium
chloride was adjusted to 20° C. The aqueous solution of sodium
hydroxide and the aqueous solution of chromium chloride were
simultaneously added into pure water adjusted to 20° C. The
addition speed was 2 ml/min for the aqueous solution of sodium hydroxide
and 4.5 ml/min for the aqueous solution of chromium chloride. The
addition was continuously performed. The addition was performed for 60
minutes. During the addition, the pH of the reaction liquid was
maintained at 7.5 to 8.5. During the addition, the temperature of the
reaction liquid was maintained at 20 to 25° C. During the
addition, the reaction liquid was stirred (700 rpm) so that the amount of
trivalent chromium was not locally excessive with respect to the amount
of sodium hydroxide. A precipitate produced by the reaction was filtered
and washed with water at 30° C., until the conductivity of the
filtrate was 1 mS/cm, to obtain chromium hydroxide. This chromium
hydroxide was suspended in pure water to obtain a slurry having a
concentration of 8%. The MV and D and degree of aggregation MV/D of the
obtained chromium hydroxide were as shown in Table 1. The solubility when
the chromium hydroxide corresponding to a Cr content of 1 g was added to
1 liter of an aqueous solution of hydrochloric acid having a pH of 0.2 at
a temperature of 25° C. (immediately after the production of the
chromium hydroxide, and after the slurry of the chromium hydroxide was
stored for 30 days) was as shown in the following Table 1.

Example 2

[0071] 59 g of a 10% aqueous solution of ammonia was used instead of the
10% aqueous solution of sodium hydroxide used in Example 1. The
temperature of the aqueous solution of ammonia was adjusted to 20°
C. Except these, chromium hydroxide was obtained as in Example 1.
Measurement as in Example 1 was performed for the obtained chromium
hydroxide. The results are shown in the following Table 1.

[0072] 70 g of a 20% aqueous solution of sodium hydroxide, and a 7%
aqueous solution of chromium chloride obtained by adding 208 g of water
to 52 g of a 35% aqueous solution of chromium chloride (manufactured by
Nippon Chemical Industrial Co., Ltd.) for dilution were each placed in a
container and prepared. Next, the aqueous solution of sodium hydroxide
and the aqueous solution of chromium chloride were adjusted to reaction
temperature shown in Table 2. Unlike Examples 1 and 2, the aqueous
solution of sodium hydroxide was added to the aqueous solution of
chromium chloride at speed shown in Table 2, while the aqueous solution
of chromium chloride was stirred. The produced precipitate was filtered
and washed with water to obtain about 12 g of chromium hydroxide. Except
these, operations as in Example 1 were performed to obtain a slurry of
the chromium hydroxide. Measurement as in Example 1 was performed for the
obtained chromium hydroxide. The results are shown in the following Table
2. However, the solubility was measured only immediately after the
production.

Comparative Example 3

[0073] Operations as in Example 1 were performed, except that the
temperature of the reaction liquid was 70° C. in Example 1, to
obtain a slurry of chromium hydroxide. Measurement as in Example 1 was
performed for the obtained chromium hydroxide. The results are shown in
the following Table 2. However, the solubility was measured only
immediately after the production.

[0074] From the above results of Examples and Comparative Examples, it is
found that the chromium hydroxide obtained by the methods in Examples has
high solubility. Particularly, as is clear from comparison between
Example 1 and Example 2, it is found that when alkali metal hydroxide is
used as the inorganic alkali used in the preparation of chromium
hydroxide, the good solubility of the chromium hydroxide is maintained
even after long-term storage.

[0075] On the other hand, it is found that in the chromium hydroxide in
Comparative Examples prepared by adding the aqueous solution of the
inorganic alkali to the aqueous solution containing trivalent chromium,
the aggregation of the primary particles is much, and the solubility is
low. It is found that even if the chromium hydroxide is prepared by
simultaneously adding the aqueous solution of the inorganic alkali and
the aqueous solution containing trivalent chromium, the primary particles
aggregate easily, and the solubility is low, in the case of high reaction
temperature (Comparative Example 3).

Example 3

[0076] Chromium hydroxide was obtained as in Example 1. This chromium
hydroxide was suspended in pure water to obtain a slurry having a
concentration of 8%. Then, each obtained slurry of the chromium hydroxide
was added and dissolved, in an amount corresponding to a Cr content of 1
g, in 1 liter of various aqueous solutions of inorganic acids at a
temperature of 25° C., or in 1 liter of various aqueous solutions
of organic acids at a temperature of 50° C. to obtain aqueous
solutions of inorganic acid chromium salts or aqueous solutions of
organic acid chromium salts. The time required for the dissolution (unit:
min) is shown in Table 3.

[0077] Chromium hydroxide was obtained as in Example 1. This chromium
hydroxide was suspended in pure water to obtain a slurry having a
concentration of 8%. Then, an amount corresponding to a Cr content of 1 g
was added and dissolved in 1 liter of aqueous solutions containing two
acids at a temperature of 25° C. to obtain aqueous solutions
containing a source of chromium (III). The time required for the
dissolution (unit: min) is shown in Table 4. The composition of the
aqueous solution of the acids used in each Example is as follows. [0078]
liquid A (pH: 0.2): 2.6% by weight of hydrochloric acid, 5.2% by weight
of nitric acid [0079] liquid B (pH: 0.4): 3.3% by weight of phosphoric
acid, 2.5% by weight of sulfuric acid [0080] liquid C (pH: 0.3): 2.6% by
weight of hydrochloric acid, 2.2% by weight of oxalic acid

[0081] A plating liquid for trivalent chromium plating having the
following composition was prepared in a square plating vessel having an
internal volume of 8 liters. Chromium plating was performed under the
conditions of a bath temperature of 50° C. and a current density
of 40 A/dm2, using a mild steel round bar as the material to be
plated, and a carbon plate as the anode. The amount of chromium consumed
and the concentration of chromium in the bath were calculated from the
weight measurement of the round bar before and after the plating. When
the concentration of chromium in the plating liquid decreased by 1 to 2
g/liter, the slurry of chromium hydroxide obtained in Example 1 was added
to the plating liquid in an amount corresponding to the electrodeposited
metal chromium, and chromium plating was continuously performed while the
plating liquid was sufficiently stirred. As a result, good chromium
plating was obtained.